There is a known wavelength monitor including a 90° hybrid that is an optical divider implemented by a multi-mode interference (MMI) waveguide having four input ports and four output ports (4×4 MMI) (see, for example, US Patent Application Publication No. 2015/0085292 and WO 2016/010528). In such a wavelength monitor, input light is split into light beams by a splitter, and the light beams are input via two waveguides with different lengths to the 90° hybrid implemented by the 4×4 MMI, which then outputs multiple light beams with different filtering characteristics.
However, among four light beams output from the 90° hybrid, only two light beams are generally used by the wavelength monitor and the other two light beams are not used by the wavelength monitor. Accordingly, with this wave monitor, a half of the input light is wasted.